Frequency measuring is usually carried out on uncoded and/or unmodulated signals, because the known measuring methods deliver wrong measuring results when applied to coded signals. For instance, document JP 09203756 discloses a network analyzer having a measuring frequency signal source which generates the desired measuring frequency by using a reference frequency given by a built-in reference frequency signal source. Signals from the frequency signal source are counted by a first counter, while signals from an external reference frequency signal source are counted by a second counter. From the two count values of the counters, a calculation control part judges the accurate frequency of the built-in signal source, thereupon controls the measuring frequency signal source, and emits signals having the desired frequency. The aim of this known device is to enhance the frequency certainty of a measuring signal using a simple circuit at reduced costs.
However, the known device fails in applications where the frequency of a signal has to be determined that is present in a signal coded and modulated form, so that it may have DC signal portions and portions where the carrier signal is oscillating with the carrier signal frequency. Such signals are widely used in the field of RFID systems where a high frequency electromagnetic signal is wirelessly transmitted from an RFID reader to RFID tags being in the transmission range of the RFID reader. The electromagnetic signal is used to carry data and instructions for the RFID tags and to energize the RFID tags when they are configured as what are called passive tags having no power supply on board. The data are usually coded in the electromagnetic signal in a serial signal coded format, for instance according to Manchester coding or Miller coding or modified Miller coding etc. The output of each of said codings is a serial bit stream containing ‘0’ and ‘1’ bits. This bit stream is used to modulate a high frequency carrier signal (e.g. having a carrier signal frequency of 13.56 MHz). Due to the modulation, the resulting signal that is transmitted by the RFID reader contains portions where the carrier signal is oscillating and DC portions (provided that the modulation depth is set to 100%). In order to ensure proper working of the RFID tags, it is necessary to continuously measure the frequency of all input signals in order to discriminate between input signals that meet the specifications and can therefore be properly processed within the RFID tags and incidental signals sent out by external signal sources which must not be processed in the RFID tag in order to prevent malfunctioning. However, due to the signal coding conventional frequency measuring methods fail, since they cannot cope with signals showing high frequency portions and DC portions of varying lengths (but still being within the specification of the RFID system). Another problem is to discriminate between permissible signal coded modulated signals and incidental signals of lower frequencies. In that case the problem is to discriminate between the DC portion of the modulated signal and the regular duty cycle of the low frequency incidental signal.